The present invention relates to a method for applying a layer of air-sprayed concrete onto a surface. In carrying out the invention, a pumpable and sprayable mixture of air-sprayed concrete is prepared from cement, aggregates, optional additives, and water. Utilizing a given mass flow of the mixture, the air-sprayed concrete mixture is fed with the aid of a concrete feed pump through a concrete feed line to a spraying apparatus. In operation there is employed a device for the introduction of propelling air which is under a feed pressure of several bars and whereby the mixture of air-sprayable concrete is sprayed onto the surface with the propelling air.
In a further aspect, the present invention also relates to a spraying apparatus with a device for the introduction of propelling air for carrying out the aforesaid method.
As used herein, the term "fillers" denotes finegrained fillers, especially flue dust. The additives can be e.g. accelerating admixtures or retarding admixtures as will be apparent to those skilled in the art. They can also be synthetic resin components. The expression "feed pressure" denotes the static pressure of the propelling air, which is supplied from an appropriate source of compressed air, or other gas at the point of introduction into the device for the charging propelling air.
Although the known techniques used in the art for accomplishing the spraying of concrete have proven themselves, there remains however, the problem that the setting or hardening time is relatively long. The pumpable and sprayable mixture of air-sprayable concrete includes excess water which is not consumed during the hydration reaction of the concrete. This increases the hardening time to a considerable degree, which adversely affects the thickness of the layer of air-sprayed concrete that can be applied during a spraying operation. In order to reduce the hardening time, the admixture of water glass to the mixture of air-sprayable concrete has previously been suggested.
However, this water glass additive has a disturbing action on the hardening process. Since water glass raises the pH of fresh concrete above the normal value of pH =12.6, undesirable alkali silicate reactions can be responsible for the fact that the firmness of the hardened concrete occasionally exhibits a falling tendency as a function of the time.